A baby goes through several stages of development, beginning as a fertilized egg. The egg develops into a blastocyst, an embryo, then a fetus.

Fertilization

During each normal menstrual cycle, one egg (ovum) is usually released from one of the ovaries, about 14 days after the last menstrual period. Release of the egg is called ovulation. The egg is swept into the funnel-shaped end of one of the fallopian tubes.

At ovulation, the mucus in the cervix becomes more fluid and more elastic, allowing sperm to enter the uterus rapidly. Within 5 minutes, sperm may move from the vagina, through the cervix into the uterus, and to the funnel-shaped end of a fallopian tube—the usual site of fertilization. The cells lining the fallopian tube facilitate fertilization.

If a sperm penetrates the egg, fertilization results. Tiny hairlike cilia lining the fallopian tube propel the fertilized egg (zygote) through the tube toward the uterus. The cells of the zygote divide repeatedly as the zygote moves down the fallopian tube. The zygote enters the uterus in 3 to 5 days. In the uterus, the cells continue to divide, becoming a hollow ball of cells called a blastocyst. If fertilization does not occur, the egg moves down the fallopian tube to the uterus, where it degenerates, and passes through the uterus with the next menstrual period.

If more than one egg is released and fertilized, the pregnancy involves more than one fetus, usually two (twins). Because the genetic material in each egg and in each sperm is slightly different, each fertilized egg is different. The resulting twins are thus fraternal twins. Identical twins result when one fertilized egg separates into two embryos after it has begun to divide. Because one egg was fertilized by one sperm, the genetic material in the two embryos is the same.

From Egg to Embryo

Once a month, an egg is released from an ovary into a fallopian tube. After sexual intercourse, sperm move from the vagina through the cervix and uterus to the fallopian tubes, where one sperm fertilizes the egg. The fertilized egg (zygote) divides repeatedly as it moves down the fallopian tube to the uterus. First, the zygote becomes a solid ball of cells. Then it becomes a hollow ball of cells called a blastocyst.

Inside the uterus, the blastocyst implants in the wall of the uterus, where it develops into an embryo attached to a placenta and surrounded by fluid-filled membranes.

Development of the Blastocyst

Between 5 and 8 days after fertilization, the blastocyst attaches to the lining of the uterus, usually near the top. This process, called implantation, is completed by day 9 or 10.

The wall of the blastocyst is one cell thick except in one area, where it is three to four cells thick. The inner cells in the thickened area develop into the embryo, and the outer cells burrow into the wall of the uterus and develop into the placenta. The placenta produces several hormones that help maintain the pregnancy. For example, the placenta produces human chorionic gonadotropin, which prevents the ovaries from releasing eggs and stimulates the ovaries to produce
estrogen
and
progesterone continuously. The placenta also carries oxygen and nutrients from mother to fetus and waste materials from fetus to mother.

Some of the cells from the placenta develop into an outer layer of membranes (chorion) around the developing blastocyst. Other cells develop into an inner layer of membranes (amnion), which form the amniotic sac. When the sac is formed (by about day 10 to 12), the blastocyst is considered an embryo. The amniotic sac fills with a clear liquid (amniotic fluid) and expands to envelop the developing embryo, which floats within it.

Development of the Embryo

The next stage in development is the embryo, which develops within the amniotic sac, under the lining of the uterus on one side. This stage is characterized by the formation of most internal organs and external body structures. Most organs form begin to form about 3 weeks after fertilization, which equals 5 weeks of pregnancy (because doctors date pregnancy from the first day of the woman's last menstrual period, which is typically 2 weeks before fertilization). At this time, the embryo elongates, first suggesting a human shape. Shortly thereafter, the area that will become the brain and spinal cord (neural tube) begins to develop. The heart and major blood vessels begin to develop earlier—by about day 16. The heart begins to pump fluid through blood vessels by day 20, and the first red blood cells appear the next day. Blood vessels continue to develop in the embryo and placenta.

Almost all organs are completely formed by about 10 weeks after fertilization (which equals 12 weeks of pregnancy). The exceptions are the brain and spinal cord, which continue to form and develop throughout pregnancy. Most malformations (birth defects) occur during the period when organs are forming. During this period, the embryo is most vulnerable to the effects of drugs, radiation, and viruses. Therefore, a pregnant woman should not be given any live-virus vaccinations or take any drugs during this period unless they are considered essential to protect her health (see Drug Use During Pregnancy).

Placenta and Embryo at 8 Weeks

At 8 weeks of pregnancy, the placenta and fetus have been developing for 6 weeks. The placenta forms tiny hairlike projections (villi) that extend into the wall of the uterus. Blood vessels from the embryo, which pass through the umbilical cord to the placenta, develop in the villi. A thin membrane separates the embryo's blood in the villi from the mother's blood that flows through the space surrounding the villi (intervillous space). This arrangement allows materials to be exchanged between the blood of the mother and that of the embryo. It also prevents the mother's immune system from attacking the embryo because the mother's antibodies are too large to pass through the membrane.

The embryo floats in fluid (amniotic fluid), which is contained in a sac (amniotic sac). The amniotic fluid provides a space in which the embryo can grow freely. The fluid also helps protect the embryo from injury. The amniotic sac is strong and resilient.

Development of the Fetus and Placenta

At the end of the 8th week after fertilization (10 weeks of pregnancy), the embryo is considered a fetus. During this stage, the structures that have already formed grow and develop. The following are markers during pregnancy:

By 12 weeks of pregnancy: The fetus fills the entire uterus.

By about 14 weeks: The sex can be identified.

By about 16 to 20 weeks: Typically, the pregnant woman can feel the fetus moving. Women who have been pregnant before typically feel movements about 2 weeks earlier than women who are pregnant for the first time.

By about 24 weeks: The fetus has a chance of survival outside the uterus.

The lungs continue to mature until near the time of delivery. The brain accumulates new cells throughout pregnancy and the first year of life after birth.

As the placenta develops, it extends tiny hairlike projections (villi) into the wall of the uterus. The projections branch and rebranch in a complicated treelike arrangement. This arrangement greatly increases the area of contact between the wall of the uterus and the placenta, so that more nutrients and waste materials can be exchanged. The placenta is fully formed by 18 to 20 weeks but continues to grow throughout pregnancy. At delivery, it weighs about 1 pound.

Stages of Pregnancy

Although pregnancy involves a continuous process, it is divided into three 3-month periods called trimesters (weeks 0 to 12, 13 to 24, and 25 to delivery).

Events

Weeks of Pregnancy

1st Trimester

The woman’s last period before fertilization occurs.

0

Fertilization occurs.

The fertilized egg (zygote) begins to develop into a hollow ball of cells called the blastocyst.

2

The blastocyst implants in the wall of uterus.

The amniotic sac begins to form.

3

The area that will become the brain and spinal cord (neural tube) begins to develop.

5

The heart and major blood vessels are developing. The beating heart can be seen during ultrasonography.

6

The beginnings of arms and legs appear.

7

Bones and muscles form. The face and neck develop.

Brain waves can be detected.

The skeleton is formed. Fingers and toes are fully defined.

9

The kidneys begin to function.

Almost all organs are completely formed.

The fetus can move and respond to touch (when prodded through the woman’s abdomen).

The woman has gained some weight, and her abdomen may be slightly enlarged.

10

2nd Trimester

The fetus’s sex can be identified.

The fetus can hear.

14

The fetus’s fingers can grasp. The fetus moves more vigorously, so that the mother can feel it.

The fetus’s body begins to fill out as fat is deposited beneath the skin. Hair appears on the head and skin. Eyebrows and eyelashes are present.

16

The placenta is fully formed.

20

The fetus has a chance of survival outside the uterus.

The woman begins to gain weight more rapidly.

24

3rd Trimester

The fetus is active, changing positions often.

The lungs continue to mature.

The fetus’s head moves into position for delivery.

On average, the fetus is about 20 inches long and weighs about 7 pounds. The woman’s enlarged abdomen causes the navel to bulge.

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